1. Field of the Invention
The present invention relates in general to methods and apparatus for exposing color cathode ray tubes to form a fluorescent surface.
2. Description of the Prior Art
In color cathode ray tubes it is conventional to provide a fluorescent surface such as a color fluorescent surface of stripe pattern wherein black stripes which comprise a light absorbing layer are formed between color fluorescent stripes of red, green and blue and can be made in a process such as described hereafter. In the prior art process, a photoresist film is first applied to an inside surface of a panel of a cathode ray tube and then dried and then an aperture grill which is a color selecting electrode with a number of beam transmission holes of slit shape which are ranged in the desired pitch is used as an optical mask and ultraviolet exposure is accomplished through the aperture grill. Then the exposed photoresist material is developed so as to form a number of resist layers of stripe shape in positions corresponding to the various colors. The ultraviolet exposure is accomplished three times, one each for the red, green and blue colors, by shifting the positions of the exposure light to the light source positions of the different colors.
Then, carbon slurry is applied to the whole surface of the tube including the resist layer and dried. Then the resist layer is lifted off together with a carbon layer above it so as to produce carbon stripes of the prescribed pattern, in other words, black stripes. A first fluorescent slurry of green color, for example, is applied thereto and exposed and then a development treatment is done so as to produce the green fluorescent stripe on the so-called blank photoresist stripe width between the prescribed carbon stripes. In similar processes, blue and red fluorescent stripes are formed in other photoresist stripes so that the intended color fluorescent surface is obtained.
In such prior art exposure methods, however, depending upon the optical dimension of the color cathode ray tube, the light intensity distribution transmitted through the slits of the aperture grill may be subject to Fresnel diffraction having a waveform distribution such as illustrated in FIG. 1A. When this occurs, so as to obtain the photoresist stripe required in the design of the cathode ray tube, in other words, with a width W of a blank photoresist stripe 3 between carbon stripes 2 as shown in FIG. 1B, the edge of the stripe of the photoresist stripe is produced at positions depending upon the derivative of .differential.I/.differential.x of the transmission light intensity distribution I where I is the transmission light intensity and which is extremely small. The derivative of the photo crosslinking distribution of the photoresist film becomes small and thus the edge becomes uneven or rough which is significant as shown in FIG. 1B and unevenness of color will be produced macroscopically which will deteriorate the quality of the color cathode ray tube.
So as to eliminate these disadvantages in a conventional method illustrated in FIG. 2 the position of the exposure light source is moved from the reference position O for the green, blue or red color laterally to positions Q1 and Q2 which are laterally offset in opposite directions from the reference position O. Then ultraviolet rays 4 and 5 are irradiated from the positions Q1 and Q2, respectively. Such exposure method is referred to as the two point light source exposure method. In such method, the transmission light intensity distribution 8 comprises the superposition of two Fresnel diffraction waveforms 6 and 7 as illustrated in FIG. 3 and the intended photoresist stripe width W is obtained therefrom. In FIG. 2 a panel 9 with an inside surface coated by a photoresist film 10 is exposed with an aperture grill 11 and a correction lens 12 is mounted between the ultraviolet exposure source and the panel 9 as shown. The correction lens approximately provides that the light path will approximate the actual travelling path of the electron beam.
In the two point light source exposure method of the prior art, however, the superposed transmission light intensity distribution 8 illustrated in dashed line in FIG. 3 is not optimized throughout the inside surface of the panel as shown by the dip in the curve in FIG. 3 at the center and this method has the following disadvantages.
Depending upon the optical dimensions of the color cathode ray tube, there may be regions in the tube where it is impossible to properly manufacture the desired stripes. Since the derivative .differential.I/.differential.x of the transmission light intensity distribution 8 becomes small in some regions of the panel inside surface, the derivative .differential.Q/.differential.x of the photo crosslinking distribution of the photoresist film becomes small and thereby the variation of the photoresist stripe width becomes significant as illustrated in FIG. 1B and the quality of the tube deteriorates. Variations caused by the materials such as the slit width of the aperture grill or the distance between the aperture grill and the panel (Bar-Height) affects directly the generation of unevenness in color and the reproduction yield of tubes becomes lowered.
FIGS. 6A through 6F illustrate the transmission light intensity distribution in solid line and the derivative of the .differential.I/.differential.x in broken line at arbitrary positions (x.sub.i, y.sub.i) on the inside of the panel surface obtained by the conventional two point light source exposure method. FIGS. 6A, 6B, 6C and 6D and 6E and 6F correspond to the center upper position (x.sub.i, y.sub.i =1, 180), the center (x.sub.i, y.sub.i =1, 1), the intermediate upper position (x.sub.i, y.sub.i =127, 180), the intermediate center position (x.sub.i, y.sub.i =127, 1), the peripheral upper position (x.sub.i, y.sub.i =255, 180) and the peripheral center position (x.sub.i, y.sub.i =155, 1) respectively. As clearly illustrated in FIGS. 6A through 6F the derivative .differential.I/.differential.x of the transmission light intensity distribution at positions corresponding to the edge of the photoresist stripe width W becomes large in the center and at peripheral positions but the derivative .differential.I/.differential.x becomes small in intermediate positions and, thus, the manufacturing becomes impossible or variations of the photoresist stripe width becomes significant at the intermediate positions.